The present invention relates to an alumina-based ceramic composition and to a substrate obtained by pressing using this composition.
At present, ceramic substrates are used in the electronics industry, particularly for producing chip-carriers or as carriers for hybrid circuits. On these substrates are formed systems of electrical connections making it possible to connect the components, such as integrated circuits, to the electrical elements of the apparatus in which the ceramic substrate is fitted. These systems of the electrical connections are generally produced in accordance with either the thick layer procedure (e.g. by a screen process printing method) or by the thin layer procedure (e.g. by a vacuum deposition method). The first of these procedures can be used in all cases, but particularly when the average roughness of the substrate is relatively high (above 10 micro-inch), whilst the second can only be used for substrates having a limited roughness. The first procedure is easier and more economic to perform, but does not make it possible to obtain fine patterns.
Among the industrial solutions for producing ceramic compositions for forming substrates, namely casting or moulding, mechanical pressing and isostatic pressing, the second is more particularly favoured as a result of its simplicity, particularly when it is a question of directly producing members which can be fritted in a furnace. Thus, as the member is directly moulded, the cutting stage which exists in both the casting process and in isostatic pressing process is avoided, as well as any possible operations involving the perforation of the member as a result of designing the mould in an adequate manner. It is merely necessary to provide in the mould, pins corresponding to the holes in the substrate. This particularly economic process does not, however, make it possible to directly produce ceramic substrates of the "thin layer" type with the well known ceramic compositions. Thus, the members obtained are much too rough and it is necessary to grind the surface of the substrate in order to make it suitable for the deposition of thin layers.
It is known from French Pat. No. 1 388 155 to use deflocculating agents in alumina-based ceramic compositions in order to make them suitable for mechanical or isostatic pressing. This patent also describes the addition of a certain quantity of binders, which ensure that the pressed object has a certain "hold" and does not deform during the fritting operation. However, the compositions described in this patent do not make it possible to produce members with a complicated shape having a limited roughness after fritting, i.e. below 10 micro-inch, without a mechanical grinding operation. It has in particular been found that the use of the different compositions described in this patent do not make it possible to obtain ceramic substrates with a limited roughness and which have holes made directly during the mechanical pressing process.